As a mass flow controller for controlling a flow rate of gas for use in, for example, a semiconductor process, there has been known a configuration as shown in Patent Literature 1.
A fluid control valve built in this mass flow controller includes: a valve seat having a valve port formed to communicate with an upstream side flow passage; a diaphragm valve body seated on the valve seat to close the valve port; and an actuator for driving the diaphragm valve body. In this configuration, in order to improve a sealing property while preventing an internal leakage of the fluid, a resin coating is applied over an entire lower surface facing the valve seat in the diaphragm valve body.
Herein, since the sealing property cannot be ensured if there is unevenness on a surface of a thin film formed by the resin coating, the surface of the thin film is required to have flatness with high precision. Therefore, it is necessary to polish the surface of the thin film by a process such as plane-lapping and the like.
However, since the thin film formed by the resin coating is thin in film thickness and low in hardness, a lapped surface formed by the plane-lapping tends to incline, and this results in a problem that the thin film may peel off and the film thickness may not be uniform due to side cutting. Therefore, even though the polishing process such as plane-lapping is performed, the sealing property is not sufficiently ensured, and also it becomes difficult to maintain durability for a long time period.